Pig-to-human xenotransplantation may alleviate the critical shortage of organs available for transplantation. However, hyperacute rejection (HAR), acute vascular rejection (AVR), and cell-mediated rejection pose significant immunological hurdles to making xenotransplantation a clinical reality. Now that pigs lacking a functional alpha-1,3-galactosyltransferase gene have been generated, HAR has been completely eliminated and AVR appears to be substantially reduced. Cell-mediated rejection (i.e., by T cells and NK cells) continues to pose a serious problem. Viruses have evolved a plethora of mechanisms to avoid T cell and NK cell recognition and destruction. The proposed project aims to exploit such "viral stealth" mechanisms for the purpose of xenotransplantation. The focus of the proposed studies will be on porcine cytomegalovirus (PCMV) since it is likely to harbor genes which function optimally in pig cells. Under Specific Aim 1, the complete genome of PCMV will be cloned and sequenced. The goal of Specific Aim 2 is to identify potential immunomodulatory open reading frames (ORFs) of PCMV, first by semi-high throughput assays, and then, for a subset of PCMV ORFs, by more detailed and refined analyses. The Phase I project will be deemed successful when we have identified specific genes from PCMV which impart stealth to pig cells in vitro when exposed to cells from the human immune system.